1. Field of the Invention
The present invention relates to a reflective display device to perform a display by using external light such as natural light, and more specifically, it relates to an illuminating structure of a reflective display device, which is used as an auxiliary when external light is scarce.
2. Description of the Related Art
Among current various modes of display devices, mainly adopted are a TN mode or an STN mode in which a nematic liquid crystal having a twisted or super twisted alignment is used. However, these modes require a pair of polarizers for operation, and because of the light absorption thereof, they have a low transmittance, incapable of achieving a bright display screen. In addition to the above modes, a guest-host mode which uses a dichroic dye has been developed. A liquid crystal display device having a guest-host mode takes advantage of the anisotropy of the absorption coefficient of the dichroic dye added to the liquid crystal, in order to perform the display. By using a rod-shaped dichroic dye, the alignment direction of the dye changes as the molecular alignment of the liquid crystal is changed by applying a voltage to the electric field since the molecules of the dye are aligned in parallel to the molecules of the liquid crystal. The dye does or does not develop a color depending on the direction, and therefore by applying a voltage, the coloring mode of the liquid crystal display device can be switched.
FIG. 5A and FIG. 5B show a HEILMEIER type guest-host liquid crystal display device. FIG. 5A shows the state in the absence of an applied voltage, while FIG. 5B shows the state in the presence of an applied voltage. This liquid crystal display device includes a p-type dye and a nematic liquid crystal having a positive dielectric anisotropy (N.sub.p liquid crystal). The p-type dichroic dye having an absorption axis which is substantially parallel to the molecular axis, strongly absorbs the polarization component Lx which is parallel to the molecular axis, and hardly absorbs the polarization component Ly which is perpendicular to it. In the state shown in FIG. 5A when no voltage is applied, the polarization component Lx included in the incident light is strongly absorbed by the p-type dye, resulting in the coloring of the liquid crystal display device. On the other hand, in the state shown in FIG. 5B when a voltage is applied, the N.sub.p liquid crystal having a positive dielectric anisotropy rises in response to the electric field and accordingly the p-type dye is perpendicularly aligned. Therefore, the polarization component Lx is only slightly absorbed, resulting in the liquid crystal display device being substantially colorless. The other polarization component Ly included in the incident light is hardly absorbed by the dichroic dye whether the state of the voltage is being applied or not being applied. Accordingly, in the HEILMEIER type guest-host liquid crystal display device, a polarizer is provided beforehand to remove the other polarization component Ly for improving the contrast.
Although the guest-host liquid crystal display device shown in FIG. 5 is a transmissive type, a reflective liquid crystal display device is also known. For example, a reflective guest-host liquid crystal display device, as shown in FIG. 6, has been proposed, in which a polarizer is removed on the side of the incident light, while a quarter-wavelength plate and a reflector are provided on the emission side. In this device, the polarization directions of the two polarizing components Lx and Ly which are orthogonal to each other are rotated by 90 degrees at both incident light and reflected light paths by the quarter-wavelength plate in order to exchange the polarizing components with each other. Therefore, in the off-state (absorption state) shown in FIG. 6A, individual polarizing components Lx and Ly are absorbed either at the incident light path or at the reflected light path. In the on-state (transmission state) shown in FIG. 6B, both polarizing components Lx and Ly are hardly absorbed. Thus, the utilization efficiency of the incident light can be improved.
In the transmissive display device shown in FIG. 5, a panel holding a liquid crystal as an electro-optical material is provided between a pair of transparent electrodes, and a light source (backlight) for supplying illumination light is arranged on the rear of the panel. The image is viewed from the front of the panel. A backlight is essential to the transmissive type, and, for example, a cold cathode fluorescent tube or the like is used. Accordingly, from the viewpoint of the display device as a whole, the backlight consumes most of the electric power, which is unsuitable for displays of portable apparatuses. On the other hand, in the reflective type shown in FIG. 6, a reflector is arranged on the rear of the panel. External light such as natural light enters from the front and the image is viewed also from the front of the panel by making use of the reflected light. Differing from the transmissive type, the reflective type does not use a light source for supplying illumination light in the back, resulting in a relatively low rate of electric power consumption, which is suitable for displays of portable apparatuses. However, in the reflective display device, the image cannot be viewed in an environment where external light is scarce, for example, at night, which remains to be a problem to be solved.